1. Introduction
The name “wax emulsion” is in some sense a misnomer, because the product is really a dispersion of solid wax particles in a continuous phase of water. The emulsion (defined as two immiscible liquids) exists only when the wax is in a molten form when the emulsion is formed at high temperature. As mentioned before, a wax emulsion should be stabilized, by either non-ionic emulsifiers (steric mechanism), ionic emulsifiers (electrostatic mechanism) or by the combination of both types to benefit with both stabilization mechanisms. Other ingredients may be added such as electrolytes, thickeners or preservatives, to help stabilize the dispersion.
It is well known that wax emulsions are extensively used as additive in many areas of applications. They can be used to control the processability of the products or to enhance the properties of the coatings used to protect surfaces from abrasive forces or moisture exposure. The fundamental properties of waxes are derived from their chemical basis, melting point and their polarity.
The size of the wax particles in emulsions is a very important consideration when developing an emulsion for a specific aim. For the modification of tactile properties, it is important that the wax particles emerge through the coating layer and this requires a particle size larger than the film thickness. However, large particles may cause film defects or product failure. On the other hand, very fine particle size ensures homogeneous incorporation in the formulation, maximizing the required effects and avoiding film defects. Moreover, high gloss finishes are maintained.
Concentrol provides specific solutions to defined problems, as we have the know-how in the production of wax emulsions. The particle size range can be controlled during the emulsification process in order to meet precise specifications. The case shown below is a nice example of particle size tuning in a polyethylene-based wax emulsion.
2. Emulsification procedure
To show the proof of concept, we have used EMULTROL EPH formulation. It is a water-based non-ionic/anionic polyethylene wax emulsion. Because of the high melting point of the wax, it can only be emulsified under pressure in a closed system. Additionally, the indirect emulsification technique was used because high quality emulsions with the greatest consistency are obtained. The pressure dilution (indirect) emulsification technique involves initially creating a high solids emulsion of the wax and surfactant by holding back a portion of the water. This allows for better and more uniform micelle formation. However, it requires two pressure vessels to manufacture.
The followed procedure is mentioned below. Polyethylene wax was charged into an autoclave reactor together with the non-ionic emulsifier, alkali, sodium disulfate and part of the water at room temperature. The autoclave was closed and heated to 125-135˚C while stirring, and the temperature was maintained for 30 minutes. The second part of the water was then injected (previously heated to 100˚C) under pressure. At this point, cooling down to room temperature was done as quickly as possible (by shock cooling) to ensure best results. A preservative should be added to aid shelf life of the emulsion.
3. Results
The surfactant type and concentration are critical aspects to consider for the size and stability of wax droplets of the continuous phase (water). In Figure 1, we can distinguish the different emulsions of EMULTROL EPH obtained by varying the amount of emulsifier. The same surfactant was used in all of them, but its concentration from A to C was progressively reduced.
Figure 1 – EMULTROL EPH with different particle size
As previously mentioned, emulsions contain both a dispersed and a continuous phase, with the boundary between the phases called the interface. Emulsions usually appear cloudy or white because light is scattered off the phase interphases between the components in the mixture. However, microemulsions and nanoemulsions having a droplet particle size of less than 100 nm, appear translucent. This property is due to the fact that light waves are scattered by the droplets only if their sizes exceed about one-quarter of the wavelength of the incident light. Since the visible spectrum of light is composed of wavelengths between 390 and 750 nm, if the droplet sizes in the emulsion are below about 100 nm, the light can penetrate through the emulsion without being scattered. This singularity can be well perceived in emulsion A from Figure 1, and is also corroborated by the measurement of the emulsion’s particle size shown in Figure 2.
Figure 2 – Particle size measurements of A, B and C emulsions respectively.
Even so, when the emulsion is diluted, short-wavelength blue light is scattered more, and the emulsion will appear bluer in colour. This phenomenon is known as the “Tyndall Effect”. On the contrary, if the emulsion is concentrated enough, the colour will be distorted to comparatively longer wavelengths and will appear more yellow. This peculiarity can be observed in Figure 3, where emulsions A and B were diluted to a 10% of its original concentration in deionized water (A’ and B’ respectively)
Figure 3 – Appearance of original emulsions A and B and their corresponding 10% dilutions, A’ and B’.
4. Benefits of controlling particle size
Emulsions are inherently unstable, and energy input is needed to form them. Over time, emulsions tend to revert to the stable state, which leads to the phase’s separation. Therefore, emulsifiers are used to reduce the interfacial surface tension and stabilize the emulsion. Microemulsions are thermodynamically stable, while translucent nanoemulsions are kinetically stable.
In varnish and paint systems the wax particles form a network which reduces the settling of matting (anti-gloss) agents, and therefore improves their matting effect. The particle size of the wax dispersion also affects the gloss of the coating.
The fine particle size can result in a lack of efficiency as a coating additive. Since the wax must protrude through the coating surface in order to have any effect, smaller particles are less likely to project beyond surface irregularities, especially in pigmented coatings. In applications where, good permeability and or transparency is a factor (for instance: fibre finish and polish applications), a very small particle size emulsion may be desirable.
5. Conclusions
The choice of emulsifier is essential in emulsions formulation. They play a key role in both emulsion formation and stabilization. Emulsifier selection is based upon the final product characteristics, emulsion preparation methodology, the chemical and physical characteristics of each phase, and the presence of other functional components in the emulsion. But once the emulsifier was selected, its content exerts a strong influence on all the emulsion properties, such as stability, droplet size distribution, surface and interfacial tension, wetting ability, viscosity, etc.
In this article we have demonstrated that emulsifier concentration is strongly related with emulsion particle size, its efficiency raises when higher amount of surfactant is used exhibiting lower particle size. For that purpose, we have used EMULTROL EPH emulsion, and by varying the emulsifier quantity, different particle sizes were obtained. Consequently, the emulsions show clearly distinguishable appearance.
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